The purpose of this study at the Olivenhain Water Treatment Plant was to determine the t10, at the nominal operating capacity of 25 million gallons per day (MGD), of a chemical injected at the raw water sodium hypochlorite injection point to the permeate (filtered water) pumps. A tracer study was performed to verify the detention time through the membrane basins. The testing method used here was based on the SPANDS method as suggested in the HACH DR/2010 Spectrophotometer Handbook. A HACH spectrophotometer (HACH DR/2010) was used to analyze water samples that were collected in vacuum-sealed Fluoride Reagent Accu Vac ampuls. The ampuls were read by the spectrophotometer at a wavelength of 580 nanometers. Theoretical data suggest that the system be dosed for a period of time equal to three times the theoretical residence time, t10, value. In order to mimic the theoretical concentration curve, a sampling regime similar to the shape of the curve and based on the "3 times the t10" value was devised. Includes figures.